mfd: Include linux/io.h to jz4740-adc

Include linux/io.h to fix below build error:

  CC      drivers/mfd/jz4740-adc.o
drivers/mfd/jz4740-adc.c: In function 'jz4740_adc_irq_demux':
drivers/mfd/jz4740-adc.c:73: error: implicit declaration of function 'readb'
drivers/mfd/jz4740-adc.c: In function 'jz4740_adc_set_enabled':
drivers/mfd/jz4740-adc.c:110: error: implicit declaration of function 'writeb'
drivers/mfd/jz4740-adc.c: In function 'jz4740_adc_set_config':
drivers/mfd/jz4740-adc.c:146: error: implicit declaration of function 'readl'
drivers/mfd/jz4740-adc.c:151: error: implicit declaration of function 'writel'
drivers/mfd/jz4740-adc.c: In function 'jz4740_adc_probe':
drivers/mfd/jz4740-adc.c:249: error: implicit declaration of function 'ioremap_nocache'
drivers/mfd/jz4740-adc.c:249: warning: assignment makes pointer from integer without a cast
drivers/mfd/jz4740-adc.c:289: warning: passing argument 3 of 'mfd_add_devices' discards qualifiers from pointer target type
include/linux/mfd/core.h:93: note: expected 'struct mfd_cell *' but argument is of type 'const struct mfd_cell *'
drivers/mfd/jz4740-adc.c:299: error: implicit declaration of function 'iounmap'
make[2]: *** [drivers/mfd/jz4740-adc.o] Error 1
make[1]: *** [drivers/mfd] Error 2
make: *** [drivers] Error 2

Signed-off-by: Axel Lin <axel.lin@gmail.com>
Acked-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>